1. Field of the Invention
The present invention relates to devices useful in determining the activity of ions in a sample of liquid solution, and is particularly useful in potentiometrically measuring ion activity in drops of biological fluids.
2. Description of the Prior Art
There is a variety of apparatus in the prior art for measuring ion concentration in solutions. Typically such apparatus includes a reference electrode and an ion-selective electrode. When both the reference and the ion-selective electrodes are immersed into a solution to be analyzed, they constitute an electrochemical cell with an electrical potential developed across the electrodes. This electrical potential is proportional to the logarithm of the activity of the ions to which the ion-selective electrode is sensitive.
An ion-selective electrode test device which is both disposable and usable with drop-sized test samples is shown in commonly assigned U.S. Pat. No. 4,053,381 which issued to D. P. Hamblem et al on Oct. 11, 1977. Two solid electrodes are mounted on a frame, and a capillary bridge is provided for promoting ionic migration between the electrodes upon application of one or more liquid drops to each electrode. The capillary bridge includes a support layer and a porous layer with ionic access to both electrodes. When a drop of liquid is applied to each electrode, the drops spread into the capillary bridge until contact is made at a thin junction interface, permitting ionic migration between the drops. Preferably a reference solution of known ion activity is applied to one (reference) electrode and the test sample solution is applied to the other electrode. The electrical potentials at the interfaces between the drops of liquid and the electrodes are measured and compared to provide an indication of ion activity in the test sample solution.
Although the device disclosed in U.S. Pat. No. 4,053,381 provides excellent results in use for determining ion activity in liquids, some problems arise with respect to the degree of care which must be exercised during operation. For example, it is highly desirable that the capillary bridge absorb a fairly exact amount of liquid. Too much absorption leaves insufficient liquid at the electrodes, while too little absorption might result in spill over onto the electrodes. Controlled absorption provides a reproducible fluid junction location and forming time within the bridge.
Another adverse situation might arise by the liquid placed in the region of the electrodes spreading, not only within the capillary bridge, but also across the top surface of the bridge. This, on occasion, may result in the formation of an apparent junction on top of the bridge, causing liquid mixing and variable readings. Such a situation will be referred to herein as "external bridging" of the capillary bridge.
Another possible cause for imprecision is evaporation of liquid from the drops. Even though evaporation of the test and reference solutions would normally occur at the same rate, changes in electrical potential due to changes in concentration would not necessarily cancel because of the presence of different interferents in the test and the reference solutions. The effect of such interferents may be amplified by evaporation.
By the present invention, we have provided an improvement in test devices such as described in U.S. Pat. No. 4,053,381, wherein absorption of the solution drops by the capillary bridge is controlled, external bridging is eliminated and evaporation losses are reduced. Additionally, test devices manufactured in accordance with the present invention exhibit exceptionally large drop placement latitude, i.e., a drop of liquid may be placed upon the device anywhere over a large area and still wet the electrode area. This feature is important in permitting relaxed manufacturing tolerances in automatic processing apparatus wherein the drops are applied mechanically.